Sizing agents are added to fine paper to improve the runnability of the paper machine and improve the performance of the paper in end-use applications. Alkyl ketene dimer (AKD, e.g., Aquapel.RTM.364 dimer available from Hercules), alkenyl ketene dimer (AlkKD, e.g., Precis.RTM. 787 dimer available from Hercules), and alkenyl succinic anhydride (ASA) are the most commonly used internal sizing agents for paper made under alkaline conditions. Traditional AKD's are made by the dimerization of two saturated, straight-chain fatty acid chlorides (e.g., palmitic or stearic acid chlorides). Liquid AlkKDs can be made by dimerization of unsaturated or branched fatty acids (e.g., oleic or isostearic acid). ASA-based sizing agents are made by the reaction of maleic anhydride with an internal olefin (typically C.sub.14 -C.sub.18). All three types of sizing agents--AKD, AlkKD, and ASA--have a reactive functional group that covalently bonds to cellulose pulp, and a hydrophobic tail that is oriented away from the pulp once the reaction has taken place. The nature and orientation of the hydrophobic tails causes the paper to repel water.
Although AKD, AlkKD, and ASA are all commercially successful, each has its shortcomings. Poor emulsion stability and deposits on the paper machine are the two most commonly cited shortcomings of ASA. ASA cannot be shipped and stored in emulsion form for long periods of time. The papermaker must make the ASA emulsion in the paper mill immediately before use. More importantly, even moderate addition levels of ASA can cause deposits on the paper machine, web breaks, and holes in the paper. ASA addition levels above 1.0-1.25 kg/metric tonne of paper general lead to unacceptable paper machine runnability and paper quality problems. Addition levels greater than 1.0-1.25 kg/metric tonne are often required to meet end-use sizing requirements, or to size paper grades made with high levels of filler.
Rate of sizing development is the most frequently cited shortcoming of AKD-based sizing agents. An extended period of curing is often required before sizing development is complete. In most cases, AKD sizing is completed by the time the paper has reached the winder. AKD-based sizing agents have also been associated with alkaline paper handling problems in high speed converting applications. Typical problems encountered with alkaline fine paper include: reduced operating speed, double feeds or jams in high speed copiers, paper welding, and registration errors on envelope folding and high speed printing equipment.
Commercial evaluations have consistently shown that liquid AlkKD's give sized paper with better handling performance than paper sized with traditional AKD's. Unfortunately, the improved paper handling performance generally comes at the expense of reduced sizing efficiency. Pilot scale testing has shown that none of the three commonly used reactive sizes--AKD, AlkKD, or ASA--can be used in size press applications without paper handling problems.
Polymeric surface sizing agents, such as Scripset 740 styrene maleic anhydride copolymer or Chromoset 600 styrene acrylate ester (available from Hercules Incorporated) have several advantages over reactive sizing agents added at the wet end. Adding the sizing agent at the size press develops sizing at the surface of the sheet where it is needed to obtain good black and white ink jet print quality, or resist the penetration of fountain solution on an offset press. Due to their high molecular weight, polymeric sizing agents generally do not cause the paper handling problems on high speed converting equipment or high speed copiers associated with reactive sizing agents. And finally, polymeric surface sizes added at the size press do not cause wet end deposit problems that can significantly reduce the productivity of the paper machine.
Sizing efficiency and cost are the most commonly cited disadvantages of polymeric surface sizing agents. On a pound-for-pound basis, AKD, AlkKD and ASA are 30% less expensive than a typical polymeric surface size. The high sizing efficiency of AKD, AlkKD, and ASA further increases the relative cost of obtaining sizing with polymeric surface sizes. All three reactive sizing agents are 50-100% more efficient than any commercial polymeric surface sizing agent on a pound-for pound basis.
Pilot paper machine testing has shown that liquid ketene multimer added at the size press offers a better balance of sizing efficiency, paper machine cleanliness, paper ink jet print quality, and paper runnability on high speed converting equipment than AKD, AlkKD, or ASA added at the wet end, or any polymeric surface sizing agent. Ketene multimer sizing agents have also shown advantages in paper grades, such as liquid packaging board, where wet strength is required.
Ketene multimer sizing agents based on aliphatic and alicyclic dicarboxylic acids are described in U.S. Pat. No. 5,685,815 and WO97/30218 and Brungardt, Riehle and Zhang 1, U.S. Ser. No. 08/601,113, filed Feb. 16, 1996, the entire disclosure of which references are incorporated by reference herein.